1) Field of the Invention
The present invention relates to an apparatus for measuring the Sulfur component contained in oil. Particularly, the invention relates to an apparatus for measuring the concentration of the Sulfur component contained in oil products such as kerosene and gasoline, and raw material of oil such as crude petroleum, or intermediate products thereof.
2) Related Art
Crude petroleum, the raw material of oil products, contains some Sulfur components; also kerosene and gasoline, which are refined products of crude petroleum, contain small amounts of Sulfur component therein. Since an allowable upper limit of the Sulfur component contained in the refined products of oil is determined by standards according to each country, it is important to measure the concentration of the Sulfur component contained in such products during or after refining the crude petroleum in order to manage or control the concentration of said Sulfur component.
There are previously known many methods for measuring the Sulfur component contained in the refined products of crude petroleum or in the refined products of oil, such as a micro coulometric titration oxidation method, an X-ray absorption method, a wavelength dispersive X-ray fluorescent analyzing system (WDS), or an energy dispersive X-ray fluorescent analyzing system(EDS).
Out of these conventional methods, a direct excitation type energy dispersive X-ray fluorescent analyzing system was well developed. According to this system, X-rays are directly applied onto a sample without changing the X-rays into monochrometric X-rays with the aid of a monochrometer, so that the sulfur component contained in oil can be easily measured on the spot, such as an oil factory, without applying any pre-operation to the sample; and it is not necessary to make the size of the apparatus large and the construction complex.
However, this type of system has a drawback in that it has a low measurement accuracy, which does not meet with the recent requirements in the measurement of the Sulfur component in oil. Therefore, it has been desired for a long time to develop an apparatus for measuring the Sulfur component contained in oil by which the Sulfur component can be measured with high accuracy.
The reason why a sufficient measurement accuracy cannot be obtained in the conventional direct excitation type energy dispersive X-ray fluorescent analyzing system will be explained below, referring to an apparatus where Molybdenum is used as a material of the target for the X-ray tube thereof. FIG. 1 shows a spectrum of energy of an X-ray radiated from the X-ray tube in which Molybdenum is used as a material of the target thereof. In FIG. 1, the energy of the X-rays radiated from the X-ray tube is taken on a horizontal axis and an intensity (brightness) thereof on a vertical axis. As shown in FIG. 1, the spectrum of the X-ray radiated from the X-ray tube is determined by a voltage for accelerating electronic beams to be applied on the target and the material of the target. When the electronic beams hit the target, characteristic X-rays of the material of the target and continuous X-rays (white X-rays) are generated from the X-ray tube. When the analysis is conducted using fluorescent X-rays, such characteristic X-rays are applied to the sample to excite an objective substance to be measured; and then fluorescent X-rays (characteristic X-rays) radiated from the objective substance are detected to know the concentration of the objective substance contained in the sample. In order to excite the objective substance in the measurement, it is necessary to apply X-rays having a greater energy than that of the objective substance in the measurement at an X-ray absorption edge thereof.
In the case that the objective substance to be measured is Sulfur, the energy at an X-ray absorption edge of the K-shell of a Sulfur atom is 2.47 keV. It is well known that if white X-rays having a peak energy of about 4.5-5 keV are applied on the Sulfur atom, the characteristic X-rays radiated from the K-shell of the Sulfur atom are excited in an effective manner. In the conventional measurement apparatus, however, the white X-rays used for exciting the characteristic X-rays have a considerably greater energy width, as shown in FIG. 1, when the electronic beams are accelerated at a voltage of 10 kV, for instance. That means during the period when the electronic beams are applied on the target, the energy around 2.31 keV, i.e. the energy of the K-shell characteristic X-rays of the Sulfur atom, is also radiated from the target, being included in the white X-rays. Therefore, when the white X-rays are applied on the sample oil, X-rays are scattered by the sample, and the scattered sample includes the K-shell characteristic X-rays of the Sulfur atoms; and such X-rays become a large background against the characteristic X-rays to be measured. Thus, if the concentration of the Sulfur component contained in the sample is low, it is difficult to measure it correctly. For instance, when the concentration of Sulfur component to be measured is several tens ppm, a large fluctuation of about 10 ppm is generated, working as the background.
That is to say, since radiations are generated as a probability phenomenon during the period when X-rays are measured (radiations are measured), a fluctuation would statistically appear in the measurement values. Therefore, if the background component is great, the background per se would statistically have a fluctuation; and if the signal generated from the objective substance to be measured is not sufficiently greater than the width of the fluctuation of the background, the signal to be detected would be varied in the fluctuation of the background. As stated above, in the conventional apparatus, when the signal obtained from the objective substance to be measured is small, it is difficult to measure the substance correctly. Therefore, in order to correctly measure a very small amount of a component contained in the sample, it is necessary to reduce the background as much as possible.